This invention relates to a material capable of storing solar energy as converted into another form of energy, and more particularly to a material which permits storage of solar energy in a converted form therein and release of the stored energy therefrom to be repeated cyclically over a long period and which otherwise permits the solar energy once stored therein as converted into another form of energy to be preserved therein for a long period and, therefore, serves advantageously as a safe storage vessel for a solar system.
As devices currently available for the conversion of solar energy into other forms of energy, there may be cited selective absorption membranes intended for use in solar heat collectors, and solar cells. The absorption membranes convert solar energy into heat and the solar cells convert it into electricity respectively. The heat and electricity produced by these devices can utilize solar energy only in the daytime during which the devices are exposed to the sunlight. For the heat and electricity to be utilized during the night time or on rainy days, the devices require additional expensive vessels for the storage of heat and electricity. Where solar energy, which is abundantly available in the summer, is desired to be harnessed and utilized for space heating and other similar purposes in the winter, these devices call for additional installation of voluminous and expensive facilities for the storage of heat and electricity. When the heat and electricity are to be preserved in such large vessels for a long time, however, these vessels inevitably suffer from considerable loss of such energies due to leakage.
There are various organic compounds which are capable of inducing their own photo-isomerization. A typical example of these organic compounds is norbornadiene which, on absorbing light, undergoes isomerization into quadricyclene. This compound is widely known to be capable of storing as much as 240 cal of energy per g owing to this photo-isomerization. Quadricyclene is a highly stable substance which undergoes hardly any reverse reaction at room temperature and exhibits a half life of as long as 14 hours when it is heated at 140.degree. C. This substance, when silver or a rhodium complex is added thereto, immediately undergoes a reverse reaction and converts itself with evolution of heat back to norbornadiene. The heat liberated during this reverse reaction can be utilized for space heating, for example. Mere preservation of quadricyclene at room temperature, therefore, warrants protracted storage of energy. In this preservation there is no need for an expensive, thoroughly insulated vessel as is used for storing energy in the form of hot water.
Norbornadiene, however, has a disadvantage that it absorbs only ultraviolet light and, therefore, induces virtually no photo-isomerization of its own but entails a side-reaction of the nature of polymerization under sunlight. Even if it is made somehow or other to induce its own photo-isomerization upon exposure to sunlight and if the side-reaction gives rise to a by-product in an amount of 1% based on the amount of norbornadiene, for example, then the amount of the by-product will cumulatively increase and reach 63% after the storage of solar energy and release of the stored energy are repeated a total of 100 cycles. Thus, this substance fails to provide a long service life.
Organic compounds which induce their own photo-isomerization have been described with respect to norbornadiene as a typical example. Generally these organic compounds, when exposed to light, absorb only specific portions of the light spectrum and, more frequently than not, entail side-reactions. Adoption of these organic compounds as media for the conversion of solar energy into other forms of energy, therefore, entails problems yet to be solved.
This invention is aimed at providing a material useful for the utilization of solar energy by making use of such an organic compound as is capable of inducing its own photo-isomerization.